Rotating shadowband pyranometer
View Patent ↗A pyranometer for measuring solar irradiance at a selected site comprises a light detector adapted to produce an output signal in response to incident solar radiation, a reversible motor having a curved shadowband attached to its output shaft, means for mounting the light detector and motor so that the shadowband is at a fixed distance from the light detector; and a motor controller that is adapted to periodically cause the motor to rotate the shadowband from one to the other of two limit positions, with the shadowband acting to momentarily shadow the light detector during its movement between its two limit positions. A datalogger stores and processes the light detector's signal output to provide a measure of total horizontal, direct normal and horizontal diffuse solar irradiance.
1. A pyranometer comprising:
a reversible electric motor having an output shaft;
a curved flat opaque band of selected width, said band having one end coupled to output shaft so that said band can rotate in time with said output shaft and so that the axis of rotation of said band is normal to said one end thereof;
a mount adapted to for attachment to a supporting structure so as to provide adjustment of said mount in a first degree;
motor-support means pivotally attached to said mount so as to provide adjustment in a second degree;
a light sensor having a light-receiving surface;
sensor support means for supporting said sensor so that the center of said light-receiving surface is substantially coaxial with the axis of rotation of said band, said sensor support means being adapted to permit adjustment of said light sensor in a third degree; and
motor control means for controlling operation of said motor so that periodically said band rotates through a selected angle from one stowed position to a second stowed position, with said stowed positions being such that said band casts a shadow on said light-receiving surface only while it is rotating.
2. A pyranometer according to claim 1 wherein said selected angle is approximately 300 degrees.
3. A pyranometer according to claim 1 wherein said band has a width such that it will cast a shadow that extends over all of said light-receiving surface when said band is disposed directly in line with the sun and said light-receiving surface.
4. A pyranometer according to claim 1 wherein said motor support means is a yoke having a base section attached to said mount and a pair of arms, and further including attachment means for coupling said motor to said arms.
5. A pyranometer according to claim 4 further including mechanical means defining a housing for said motor, and further wherein said attachment means comprises a bracket and said housing is attached to said bracket.
6. A pyranometer according to claim 5 wherein said sensor support means is attached to said bracket.
7. A pyranometer according to claim 5 wherein said sensor support means is pivotally attached to said bracket so as to provide said adjustment in a third degree.
8. A pyranometer according to claim 1 wherein said motor control means comprises a circular disk attached to said output shaft, a magnet mounted to said circular disk in fixed angular relation to said band, a first magnet-responsive detector mounted so as to (a) sense said magnet when said band approaches said first stowed position and (b) generate a control signal that causes said motor to stop with said band in said first stowed position, and a second magnet-responsive detector mounted so as to (a) sense said magnet when said band approaches said second stowed position and (b) generate a control signal that causes said motor to stop with said band in said second stowed position.
9. A pyranometer according to claim 8 wherein said light sensor produces an output signal in response to light impinging on said light-receiving surface, and further including means for sampling said output signal a selected number of times as said band moves from one to the other of said first and second stowed positions, whereby to obtain a stream of irradiance data.
10. A pyranometer according to claim 9 further including means for processing said irradiance data to measure the horizontal diffuse irradiance Dif h .
11. A pyranometer according to claim 9 further including means for determining the direct normal irradiance Dir n according to the equation T h =Dif h .+Dir n cos(Z), where Z is the sun's zenith angle.
12. A pyranometer comprising:
a motor support member mounted for rotational movement on a first vertical axis;
a motor unit attached to said motor support member for rotational movement on a second horizontal axis, said motor unit comprising a reversible electric motor having an output shaft;
a shadow band having a selected width and first and second opposite ends, said first opposite end being attached to said output shaft so that said shadow band will rotate with said output shaft, said band being curved between said first and second opposite ends with its curvature extending through an angle of about 133 degrees;
a sunlight sensor having a light-receiving surface;
sunlight sensor support means spaced from said output shaft but supporting said sunlight sensor so that it is located in line with the axis of rotation of said output shaft, with the distance between said sunlight sensor and said band being substantially the same along the full length of said band;
motor control means for controlling operation of said motor so that periodically said band rotates through a selected angle of approximately 300 degrees from one stowed position to a second stowed position, with said stowed positions being such that said band casts a shadow that fully occludes said light-receiving surface only once as it rotates between stowed positions.
13. A pyranometer according to claim 12 wherein said motor support member comprises two arms, and further including means connecting said motor unit to said arms so said motor unit can pivot on a selected pivot axis relative to said arms, said pivot axis extending at a right angle to axis of said output shaft.
14. A pyranometer according to claim 12 wherein said sunlight sensor can pivot on an axis that extends at an angle to said selected pivot axis of said motor unit and also the axis of rotation of said shadow band.
15. A pyranometer according to claim 14 wherein said motor control means comprises a magnet mounted for rotation in synchronism with said output shaft, said magnet having a fixed angular relationship with said shadow band, first and second Hall effect sensors mounted in position to sense said magnet and generate a control signal when said shadow band has moved into said first and second stowed positions respectively, and means for stopping said motor in response to a control signal from said first or second Hall effect sensor.
16. A pyranometer according to claim 15 wherein said light sensor produces an output signal in response to light impinging on said light-receiving surface, and further including means for sampling said output signal a selected number of times as said band moves from one to the other of said first and second stowed positions, whereby to obtain a stream of irradiance data.
17. A pyranometer according to claim 16 further including means for processing said irradiance data to measure the horizontal diffuse irradiance Dif h .
18. A pyranometer according to claim 16 further including means responsive to said output signal for determining the direct normal irradiance Dir n according to the equation T h =Dif h .+Dir n cos(Z), where Z is the sun's zenith angle.